1. Field of the Invention
The present invention is generally directed to a method and system for power de-rating reduction and, more specifically, to a method and system for power de-rating reduction in a transmitter.
2. Description of the Related Art
In general, coded orthogonal frequency division multiplexing (COFDM) systems support high data rate wireless transmission using orthogonal channels, which offer immunity against fading and inter-symbol interference (ISI) without requiring implementation of elaborate equalization techniques. Typically, COFDM systems split data into N streams, which are independently modulated on parallel spaced sub-carrier frequencies or tones. The frequency separation between sub-carriers is 1/T, where T is the COFDM symbol time duration. Each symbol may include a guard interval (or cyclic prefix) to maintain the orthogonality of the symbols. In general, COFDM systems have utilized an inverse discrete Fourier transform (IDFT) to generate a sampled (or discrete) composite time-domain signal. One undesirable attribute of COFDM systems is that they may exhibit relatively large peak-to-average power ratio (PAR), when signals from different sub-carriers add constructively. A large PAR (and/or large cubic metric (CM)) is undesirable as it requires a large dynamic range for a digital-to-analog (D/A) converter implemented within a transmitter of a COFDM system. Consequently, the converter may be used inefficiently as most sub-carrier amplitudes use a fraction of the range of the converter.
In a typical implementation, the output of the D/A converter is filtered before being applied to a power amplifier. As power amplifiers tend to be non-linear, in-band distortion and spectral spreading (or spectral regrowth) may occur. As is known, spectral regrowth may occur when a band-limited time-varying (non-constant) envelope signal is passed through a non-linear circuit. One technique for addressing non-linearity of a power amplifier has operated the power amplifier at a relatively large output power backoff (OBO). Unfortunately, operating a power amplifier at a relatively large OBO (or power de-rating) reduces the power efficiency of the amplifier. For example, at a 6 dB OBO, a power amplifier may exhibit a fifty percent (or more) loss in efficiency. To reduce the PAR and/or CM of COFDM systems, various designers have also implemented or proposed hard limiting (or clipping) directly on the signal to be transmitted. Unfortunately, directly clipping the signal to be transmitted may cause undesirable spectral regrowth and inter-user interference (or inter-carrier interference (ICI)) in systems that utilize multiple access mode.
Discrete Fourier transform-spread orthogonal frequency division multiplexing (DFT-SOFDM) has been proposed as the modulation technique for the uplink of evolved-universal terrestrial radio access (E-UTRA). Single carrier transmission schemes, such as DFT-SOFDM, generally facilitate further power de-rating reduction through the use of, for example, specific modulation or coding schemes, or clipping and spectral filtering of a signal to be transmitted. Moreover, the PAR and CM of a basic DFT-SOFDM (or single carrier-frequency division multiple access (SC-FDMA)) system is generally reduced, as compared to the PAR and CM of a basic COFDM system. To further reduce the PAR and CM of basic DFT-SOFDM transmitters, it has been proposed to pre-process an input signal prior to performing a fast Fourier transform (FFT) on a group of symbols associated with the input signal. Following this approach, selected input symbols and/or bits may be attenuated in order to reduce the PAR and CM at the output of an inverse fast Fourier transform (IFFT) of the DFT-SOFDM system.
FIG. 1 depicts a relevant portion of a prior art SC-FDMA system 100 that implements, for example, a quadrature phase shift keying (QPSK) modulation scheme. As is shown, data from a serial-to-parallel converter 102 is applied to a plurality of bit-to-constellation mapping blocks 104, which mapped the data bits to IQ bits. For example, in a system that implements a QPSK modulation scheme, each group of twenty bits would be mapped to a group of ten symbols, i.e., two bits per symbol. As another example, in a system implementing an 8-QSPK modulation scheme, each group of thirty bits would be mapped to a group of ten symbols, i.e., three bits per symbol. In the system 100 of FIG. 1, outputs of the mapping blocks 104 are provided to inputs of an M-point fast Fourier transform (FFT) block 114 and to inputs of an attenuator block 106. Outputs of the M-point FFT 114 and M-zero values are also provided to inputs of a 2M-point inverse FFT (IFFT) block 116. Selected outputs of the 2M-point IFFT block 116 are provided to a peak detector 118.
As is shown in FIG. 1, outputs of the peak detector 118 are provided to control inputs of the attenuator block 106, which attenuates selected bits (or symbols) a desired amount according to signals provided at the control inputs of the attenuator block 106. Outputs of the attenuator block 106 are provided to a conventional SC-FDMA transmitter (including an M-point FFT block 108, an N-point IFFT block 110 and a cyclic prefix (CP) block 112, which adds a desired guard band to each symbol group). It should be noted that the sub-carrier mapping block that maps the samples of the M-point FFT to N sub-carriers (in a localized or distributed manner) is not separately shown in FIG. 1. While the system 100 exhibits improved power de-rating reduction over conventional SC-FDMA systems, it would be desirable to provide further power de-rating reduction than can be readily achieved by the system 100 of FIG. 1. What is needed is a technique for power de-rating reduction for a transmitter (e.g., an uplink transmitter) that further reduces inter-carrier interference and spectral regrowth.
In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced.